assistant: Require user to accept TOS for cloud provider (#16111)

This adds the requirement for users to accept the terms of service the
first time they send a message with the Cloud provider.

Once this is out and in a nightly, we need to add the check to the
server side too, to authenticate access to the models.

Demo:


https://github.com/user-attachments/assets/0edebf74-8120-4fa2-b801-bb76f04e8a17



Release Notes:

- N/A

crates/collab/src/db/tables/user.rs

@@ -18,6 +18,7 @@ pub struct Model {
     pub connected_once: bool,
     pub metrics_id: Uuid,
     pub created_at: DateTime,
+    pub accepted_tos_at: Option<DateTime>,
 }
 
 #[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]